1. Field of the Invention
This invention relates to a method of fabricating complex composite preforms.
2. Brief Description of the Prior Art
Resin transfer molding (RTM) is a procedure for fabricating advanced composites wherein fibers in some form are placed in a tool, such as a mold, and a thermosetting resin is then injected into the tool around the fibers and hardened to provide the complex composite preform in the shape of the tool. A sub-process critical to the resin transfer molding (RTM) process is the fabrication of dry fiber xe2x80x9cpreformsxe2x80x9d. To fabricate these preforms requires the ability to hold layers of woven or directional dry, fiber or mat fiberglass(trademark) (a grass fiber), quartz, graphite, kevlar (trademark) (aramid fiber), etc.) together to form a near net shape of the composite part. This preform is then placed in a matched metal tool and injected with a properly degassed and stoichiometrically mixed thermosetting resin blend.
A typical prior art process is shown in U.S. Pat. No. 5,080,851 wherein there is disclosed a method for forming stabilized preforms for complex composite articles using a solid meltable uncatalyzed thermosetting resin as the stabilizing agent. The resin is applied to each layer of the preform and heated to melt the resin and bond the layers together. After cooling, the resin resolidifies, but does not cure and thereby holds the layers in the desired preform shape. The layers can be first formed into a flat sheet as a raw material from which shaped preforms can be made by cutting the sheet into sections. These sections are placed over a mandrel and reheated to remelt the semi-solid resin and allow the preform to conform to and thereafter retain the shape of the mandrel. Utilizing this process allegedly allows complex composite parts to be easily preformed and assembled prior to placement in a mold for resin transfer molding without inhibiting part strength because the resin is compatible with the injection resin and is cocatalyzed by the catalyst contained in the injection resin.
A problem with the prior art is that it includes thermoplastic non-reactive binders and requires stitching, both diminishing the critical properties required from an advanced composite. The prior art also includes a powdered uncatalyzed resin, distributed by hand. The distribution is operator dependent and the fact that the resin is uncatalyzed changes the reaction rate or stoichiometry. To compensate, the RTM resin must contain excess catalyst to balance out the approximately 5% uncatalyzed resin already on the preform. The inclusion of the catalyst in the unreacted RTM resin mixture reduces the pot life of the mixed RTM resin, pot life being a critical processing parameter. The prior art also uses a solid resin system to prevent caking and reflow melting to provide greater ease of application and handling. This results in a more brittle preform which will shear the layers of fabric apart if stressed.
In accordance with the present invention, there is provided a method of making composite preforms which overcomes or at least minimizes the problems inherent in the prior art. The process in accordance with the present invention is fully reactive and does not degrade critical properties of the composite. Furthermore, the catalyst is in the binder preform resin, unreactive with the binder resin until introduction of the RTM resin, and not stoichiometrically sensitive as in the prior art. The application method preferably uses controlled robotic machines, thereby improving the consistency and quality of the binder resin distribution. The melt viscosity and volumetric delivery of the preform binder resin are controllable by current technology. The present invention utilizes a resin which is semi-solid at room temperature and which holds the preform in the desired shape.
Briefly, dry fibers, which may be in the form of a woven or unwoven fabric, mat or unidirectional tape, preferably on a roll, preferably but not necessarily having a thickness of from about 4 to about 11 mils, are provided and made available for application of a catalyzed thermosetting resin which is semi-solid, highly viscous and acts as a thermoplastic. If the fibers are used in the unwoven state, they are generally positioned in the direction wherein such directionality is required. The fibers are preferably dry and have the properties of compression, tensile strength (ultimate maximum stress) and tensile modulus (ratio of stress to strain in linear elastic region), Poisson""s ratio, density and elongation, such as, for example, glass, quartz, graphite or Kevlar(trademark) fibers. The fibers can be stationary and sprayed, preferably uniformly, with a predetermined amount of resin and catalyst or can be sprayed with resin and catalyst at a controlled rate while the fibers are moving at a controlled rate so that the amount of resin on the fabric is controlled. The amount of resin deposited on the fibers is from about 5 to about 10 percent by weight of the fibers, the amount of resin used being as small as possible consistent with securing the fibers together in the final composite. The resin is a non-brittle catalyzed reactive semi-solid at room temperature thermosetting resin which is tacky at room temperature with thermoplastic properties, preferably an epoxy and preferably bisphenol A epoxy resin, such resin generally having a molecular weight from about 300 to about 400, with a catalyst in sufficient amount to be capable of catalyzing the spray applied binder resin and the RTM resin when mixed together, the catalyst being preferably cobalt naphthanate, cobalt acetylacetonate, copper acetylacetonate or zinc octoate. The ratio by weight of catalyst to resin is from about 60 to about 150 parts per million. A true catalyst is added to the resin system to maintain the reactivity rate when the final resin blend is introduced into the tool, such as a mold. This small percentage of resin with catalyst is heated with moderate amounts of heat sufficiently high to permit the resin to flow, and thermosetting will not take place (generally below 150xc2x0 F.) on the layers of fabric to bond the resin to the fabric. The catalyst in the tackifier resin becomes active later in the process and only when all the components of the RTM resin are present, the RTM components being in this case (1) the epoxy resin, (2) the polycyanate and (3) catalyst. The fibers can then be stacked or rolled up in a roll with intervening release liner if in roll form initially for later use.
The process continues wherein like patterns or sheets of the fibers coated with catalyst-containing resin are then cut from the roll and made into sheets if not already in sheet form, stacked onto each other and placed in a tool, preferably in the form of a mold. Pressure and/or heat are used to form the resin and catalyst-coated fiber to the mold shape. A hot press, such as, for example, an iron, is applied to the fiber, pressing the fiber to the mold shape with heat to melt the resin which acts as a thermoplastic, viscous liquid at this time. The resin-coated fiber sheets can be placed in the mold and pressured and/or heated therein over each other one or more at a time up to all of the sheets at one time and all the sheets in the mold are pressured and/or heated together to provide a preform. This preform retains the shape of the mold when cooled, this specifically being the shape of the desired part. The use of a semi-solid resin allows the preform to be handled without shearing of the layers of fabric, unlike the brittle solid resin systems of the prior art. Multiple layers of fabric can be preformed using this tacking method without affecting the RTM process or finished part properties. The mold is then closed and a polycyanate and an epoxy resin or a polycyanate resin alone which are or can be made flowable into the mold and wherein the epoxy resin can be the same or different from the epoxy resin initially sprayed on the fibers, are then injected into the mold to cross-link and cure with the resin and catalyst previously deposited onto the fibers. The ratio of epoxy resin to polycyanate by weight is from a finite amount approaching zero to about 60 percent and preferably from about 5 to about 60 percent by weight or a 100 percent amount of polycyante resin can be used. The shaped, cured composite is then removed from the mold.